1. Field of the Invention
The present invention relates to apparatus and method for treating a flow of material by an electric arc. The invention is concerned in particular with a method and apparatus for treating a flow of gaseous material by passing the material through an arc to form an electrical plasma for generating (in a ground or excited state) free radicals and/or atoms, and charged species, in a jet of gas.
2. Description of the Prior Art
It is known to provide apparatus for producing an electrical plasma sometimes known as a plasma torch, in which a cathode and an anode are provided within a housing and define an annular space through which a gaseous feed stock (a gas, vapour, suspension or other gaseous material) is passed, and a plasma is produced by striking an arc across a gap between the cathode and anode. Where the gaseous material passes through the gap under pressure and emerges in a stream of material, the plasma takes the form of a jet extending from the arc region and sometimes referred to as a plasma jet. In one known form of such a plasma torch, the cathode comprises a rod and the anode comprises a cylindrical structure with a frusto-conical surface extending inwardly at one end of the cylinder, the cathode rod and the anode cylinder being coaxial, with the tip of the cathode rod positioned inside the region surrounded by the frusto-conical surface. The end of the cathode is initially pointed or dome-shaped and the arc is struck between the tip of the cathode rod and a region on the frusto-conical surface of the anode. At the center of the frusto-conical surface of the anode is an aperture leading along the axis of the anode cylinder and through the anode for exit of the gaseous material which passes through the region where the arc is struck. The place on the cathode from which the arc is struck is sometimes known as the cathode root and the place on the anode at which the arc is struck is sometimes known as the anode spot.
It is known to provide an annular magnet around the cathode-anode axis for providing a magnetic field in the region of the cathode-anode gap. The magnetic field is arranged in such a manner as to cause the arc to move in a generally rotary manner around the longitudinal axis of the cathode and anode, and in this movement the anode spot travels in a generally circular path around the frusto-conical surface of the anode. However the cathode root does not move in any organised manner on the cathode and moves erratically between positions which give the minimum anode to cathode spacing for the arc. In some cases the cathode root may remain at a position until the cathode surface at that spot is sufficiently eroded by the arc to force the arc to jump to another position which again gives a minimum cathode to anode spacing for the arc. Thus in some cases the manner of operation of the arc may be that the cathode root makes intermittent, random jumps to new cathode root positions after damage has been inflicted on the cathode by erosion at the previous cathode root position.
The present invention is particularly concerned with the more efficient use of a plasma jet for generating (in a ground or excited state) free radicals and/or atoms, and charged species, in a jet of gas.
The most sophisticated plasma jets used in previous work have used a pointed cathode (usually of thoriated tungsten) surrounded by a water cooled anode (usually Cu, brass or steel), and were provided with a circumferential solenoid to form an electromagnet. The axial component of the magnetic field produced by the latter interacted with the radial component of the current, causing the D.C. arc to rotate at high speed about the central cathode. This helped to heat the gas more uniformly, to generate swirl and turbulence, and to minimise anode erosion.
However, such use of a magnetic field does not cause the cathode root to move in any organised fashion. Melting tends to occur at the pointed tip of the cathode and the achievement of a compromise between the danger of rapid consumption of the cathode and the need to maintain the cathode root at a sufficient temperature for plentiful electron emission is made difficult in the case of some gases by electron attachment, by the absorption of much energy during dissociation, and by other factors. It has been the practice in previously known methods to include in the feed stock material an admixture of large amounts of argon or other suitable monatomic gas additives.